When to Use a Semicolon before a Conjunction
 
When a sentence is made up of two independent clauses joined by a conjunction (e.g., and, or, but), it is possible to precede the conjunction with a semicolon if either of the clauses contains a comma(s).

This is quite an outdated practice, but it can be used to improve readability.

For example:

  • Mark, 17, joined the Army; and Paul, Mark's younger brother, joined the Marines.
 

Using a Semicolon before And, But, and Or

When a conjunction (e.g., and, but, or) merges two sentences into one, it should be preceded by a comma. (This is covered in the lesson on commas before conjunctions.)

However, if one (or both) of the sentences (or independent clauses as they're really called) already contains a comma (or commas), then a semicolon can be used instead of a comma to outrank the commas within those clauses.

(Note: Using a semicolon in this way is quite an outdated practice. However, you can still use a semicolon if you think it makes things clearer for your readers.)

Examples:

  • In the '60s, there were dozens of buzzards along the 7-mile trek; but, due to the decline in vermin, only 2 adults live in the area at present.
  • (This sentence is made up of two independent clauses: In the '60s, there were dozens of buzzards along the 7-mile trek. + Due to the decline in vermin, only 2 adults live in the area at present.
    As these clauses both contain commas, it is possible to use a semicolon before the conjunction that merges them into a single sentence. Remember though, it would be far more common to use a comma and not a semicolon.)

  • Mark, Dawn, and Sally adore boiled spare ribs; but Julia, a staunch vegetarian, leaves the room when they are on the menu.
  • (Here, it is acceptable to use a semicolon before but. However, using a comma would be far more common.)
 
WHAT IS A COMPOUND SENTENCE?

A sentence made up of two independent clauses is called a compound sentence. The conjunction that joins the two halves of a compound sentence should be preceded by a comma or possibly a semicolon.
 

See also:

What are conjunctions?
What are independent clauses?
Semicolons before transitional phrases (e.g. however)
Semicolons in lists
Semicolons to extend a sentence
Types of sentence